A conventional image quality correcting circuit, as is shown in FIG. 1, is designed so that the average image level (APL) for every 1 frame (or 1 field) is detected by an average value computer 10, and a corresponding correcting data is read out from a ROM 14 according to the APL, which serves as an address, in order to correct the inputted video signal, for being outputted from an output terminal 18, by an image quality corrector 16 according to an input/output conversion characteristic curve corresponding to the correcting data. The APL is obtained, for example, by dividing the sum of the products of the total number of display dots of each frame (or each field) and the occurrence frequency (number of times of occurrence) distribution of each luminance level by the total number of the dots.
However, according to the conventional case as is shown in FIG. 1, since the image quality correcting data is based on the APL, the display quality of the image can be improved where the brightness is evenly distributed in a given image but cannot be improved according to the content of the image because of a problem, that is, the lack of the consideration of the luminance level histogram (the luminance level occurrence frequency distribution).
For example, assume that there are case 1 of the luminance level occurrence frequency distribution where the luminance levels on the bright side are predominant as shown in FIG. 2(a), and case 2 of the luminance level occurrence frequency distribution where the luminance levels on the dark side is predominant as shown in FIG. 2(b). Assuming that the APL's are equal irrespective of the different luminance levels, there occurs a problem that, in the case shown FIG. 2(a), the resolution on the bright side becomes poor, while, in the case shown in FIG. 2(b), the resolution on the dark side becomes poor. Especially, as shown in FIG. 3, in the case where the luminance level occurrence frequency distribution is predominant in a narrow area on the dark side where the luminance level is low, this gives rise to a problem that the correcting characteristic curve tends to have an extremely inclined portion, causing the brightness of the image to become greater than necessary and the resultant decline of the resolution of the image on the bright side. The same is true of the case where the luminance level occurrence frequency distribution is predominant on the brighter side of the image.
In order to resolve the above-mentioned problems, the present applicant has already proposed a video signal correcting circuit as is shown in FIG. 5 under the Laid-Open Japanese Patent Application No. 8-23460. According to the proposed circuit, the inputted video signals SO, comprising analog R (Red), G (Green) and B (Blue) signals, are converted into digital R, G and B signals through an A/D 20 (Analog/Digital converter) and inputted, as a lower-rank address, to a ROM 22 (Read-Only Memory) for the input/output conversion, that is, the tone correction by the table look-up method. On the other hand, the Y-signal (luminance signal) is generated from the analog R, G and B signals by a matrix circuit 24, and the Y-signal is converted into a digital signal by the A/D 26 for input to a histogram circuit 28. The histogram circuit 28 counts the luminance level occurrence frequency (distribution) for each of the luminance level range divided into plural ranges (e.g., 4 ranges). A decoder 30 decodes the result of the count by the histogram circuit 28 for input, as a higher-rank address, to the ROM 22 for selecting the tone correcting characteristic data previously stored in the ROM 22, thereby correcting the tones of the inputted digital R, G and B signals to be outputted as the digital R, G and B signals S1.
With the video signal correcting circuit shown in FIG. 5, tone correction according to the occurrence frequency distribution of the luminance level of the inputted video signal can be made, but such a circuit still has a problem that the correcting characteristic corresponding to the occurrence frequency of each luminance level cannot be obtained.
The present invention is made in order to solve the above problems of the prior art and is intended to provide an image quality correcting circuit applicable for the correction of all kinds of images based on a best correcting characteristic corresponding to the occurrence frequency of each luminance level.